1. Field of the Invention
The present invention relates to the regulation of the molecular weights of the poly(dichlorophosphazenes) and of the poly(organophosphazenes) produced therefrom.
2. Description of the Prior Art
The poly(organophosphazenes) constitute a vast category of polymers, all of which include a skeleton or backbone comprising an --N.dbd.P--recurring structural unit onto which organic groups of various types, such as alkoxy, aryloxy, sulfur, amino, etc., have been grafted. This diversity in structure provides the poly(phosphazenes) with a particularly wide range of potential applications, including, inter alia, use as flame-retardant insulating or padding foams, flame-retardant coatings, special elastomers, conductive polymers, and biomedical appliances.
The entire range of these poly(phosphazenes) can only be prepared on an industrial scale from a common intermediate, namely, poly(dichlorophosphazene). The conversion of this intermediate to a poly(organophosphazene) is carried out by substituting the chlorine atoms by an organic substituent according to the following reaction scheme: ##STR1## where R is any desired organic radical or group, in particular those indicated above, and Y is a metal, typically an alkali metal, or a hydrogen atom.
Irrespective of the type of substitution, it is necessary to have a poly(dichlorophosphazene) starting material, the molecular weight of which is adapted to the desired final application. It is therefore necessary to control the level of such molecular weight.
To date, three routes for the preparation of poly(dichlorophosphazene) have been described. Allcock and Kugel (U.S. Pat. No. 3,370,020) describe heating hexachlorocyclotriphosphazene until about 50% of this precursor has been converted into the polymer. In this manner, the poly(dichlorophosphazene) contains only a small proportion of crosslinked gel. It is essentially present in linear form and is thus soluble, permitting it to be converted by substitution, as follows: ##STR2##
It is not possible by this process to terminate the chain growth at the desired level; therefore, the molecular weights obtained are random and, in general, very high.
A different process for preparing poly(dichlorophosphazene) entails polycondensing oligomers of the Cl.sub.3 P--(N.dbd.PCl.sub.2).sub.x --Cl.sup.+ PCl.sub.6.sup.-- type in the presence of ammonium chloride (Hornbacker and Li, U.S. Pat. No. 4,198,381), but this procedure provides only very low molecular weights.
Most recently, a process has been described which entails polycondensation of N-dichlorophosphoryl-P-trichlorophosphazene (De Jaeger, Heloui and Puskaric, French Patent 2,466,435) according to the following scheme: ##STR3##
This process produces no crosslinked gels whatsoever, making it possible to carry out the polycondensation up to complete conversion, the chain growth being effected gradually as a function of the residence time. The level of molecular weight can thus be precisely controlled by varying the duration of the polycondensation.
In French Patent 2,571,710, De Jaeger, Pagniez and Potin describe carrying out at least the polycondensation stage in a medium in which both the monomer and the polymer are soluble, with the goal of obtaining high degrees of polycondensation in a reproducible manner.
It is possible to ensure that the molecular weights are regulated by adding a chain-limiting agent to the polycondensation product (Pagniez, Passimourt, Potin, French Patent 87/09,406). However, it should be appreciate that this method of regulation by using a limiting agent essentially provides only low or medium molecular weight levels.
Allcock and Gurdner (U.S. Pat. No. 3,917,802) describe a process by means of which the molecular weight of the poly(dichlorophosphazene) can be reduced. The process entails heating this polymer in the presence of phosphorus pentachloride, causing scission of the P-N bonds in the polymer chains. This method has the drawback of requiring a reactant which is very corrosive and difficult to handle, since it is present in the form of solid lumps.